Tunable waveguide short



April 1, 1958 s. R. HENNIES ETAL TUNABLE WAVEGUIDE SHORT Filed Dec. 24,1953 IE I [5 1 INVENTORS Sruxazr 9Q HE/wv/Es UAcK A.5EOWN u I I I l l II u "I n f ATTORNEY 2,829,352 I w TUNABLE WAVEGUIDE SHORT Stuart R.'Hennies and Jack A. Brown, Palo Alto, Calif.,

assignors to Varian Associates, San Carlos, Califl, a

corporation of California 7 Application December 24, 1953, Serial No.400,308

Claims. Cl. 333-98 This invention relates generally to adjustable meansfor shorting waveguides and the invention has reference, moreparticularly, to a novel tunable waveguide short suitable for tuningconnected resonant circuits such as the resonant cavities of klystrontubes.

Heretofore it has been common to employ movable plungers with springfingers engaging in the walls of a 2,829,352 9? e r? 1, we.

2 H shown a waveguide 1 of rectangular cross-section within which iscontained a novel tunable waveguide short 2 of this invention. Thisshort comprises essentially a transverselyextending piston member 3 ofrectangular crosssection and somewhat smaller in dimensions than theinner dimensions of the waveguide 1. The piston 3, which is of aconducting material such as copper or brass, is shown provided withupper and lower transversely extending bearing members 4 and 4 made ofinsulating material such as Teflon or polystyrene. 'Ihese bearings v areset into and retained within recesses 5 and 5 provided waveguide foreffecting the short circuit of the guide at One object of the presentinvention is to provide a novel tunable waveguide short that is ofrelatively inexpensive construction and especially suitable for theadjustable shorting small sizes of waveguide, the said novel shortemploying spring contact means that will not deteri orate with timesAnother object of the present invention is to provide a novel tunablewaveguide short comprising essentially a coiled spring carried by amovable plunger, said spring being so disposed within the guide thatopposite loops thereof engage opposite walls of the waveguide, thespring being preferably under slight compression so that a firm andpermanent contact is made both at the upper and lower walls of theguide.

. Another object of the present invention is to provide a tunablewaveguide short that is especially suited for tuning cavity resonatorssuch as those employed in klystron tubes thereby varying the outputfrequency of the tube.

Other objects and'advantages of this invention will become apparent fromthe specification, taken in connection with the accompanying drawingswherein the invention is embodied in'concrete form.

In the drawings,

Fig. l is a perspective view with parts broken away showing the noveltunable waveguide short of this inven tion mounted in a waveguide,

' Fig. 2' is a fragmentary section view illustrating the short employedin connection with tuning a reflex klystron,

' Fig. 3 isi'a plan view with parts broken away showing a slightlymodified construction of short,

Fig. 4 illustrates a further modified construction of short especiallyadapted for circular waveguides,

Fig. 5 is an end view of the construction'shown in Fig. 4, and

Fig. 6 illustrates another modification.

Similar characters of reference are used in the above figures todesignate corresponding parts.

Referring now to Fig. l of the drawings, there is in the upper and.lower surfaces of the piston 3. An adjusting rod 6 extendslongitudinally of the guide 1 through the end wall'7 thereof and isattached to the piston 3' as by being threaded thereinto. By moving therod 6'longitudinally the connected piston 3 is moved within the guide 1,the bearing members 4 and 4 serving to position the piston 3 accuratelywithin the guide without the metallic portions of the piston actuallytouching the guide.

The piston 3 is provided with a forward central lip extension 27substantially midway of the height thereof which lip is provided with aseries of mutually spaced apertures 8 arranged in a transverse line. Acoil spring 9 of conducting material is threaded through the apertures 8and is'of such dimensions as to press both against the'upper and lowerinner walls of the guide 1 thereby making good electrical contact withthese walls of the guide. Owing to the elasticity of the material ofspring 9and to the fact that the loops of the spring extend completelyacross the interior of the guide the upperand lower portions of theseloops bear firmly against the walls of the guide and do not lose thisresilient bearing pressure regardless of the age of the short which isnot true of those types of shorts employing spring fingers to theforwardedge of the bearings 4 and 4' may be made substantially equal to aquarter wave length of the operat-' ing frequency of the guide in whichcase the bearings 4 and 4' can be made substantially a quarter wavelength in width and similarly the portion of the plunger of the piston 3to the rear of the bearings 4 and 4" can be made substantially a quarterwave length long thereby forming a low pass filter having three sectionsserving to prevent any outward escape of energy beyond the piston whichmight tend to set up undesired reflections. In some instances, however,the use of a low pass filter is unnecessary inasmuch as the spring 9provides a very effective short circuit.

'Fig. 2 shows such a tunable short employed for tuning the cavity of aklystron 10 of the reflex type, the said to the use of the simple spring9 the short of the present invention can be made in very small sizes toaccommodate the .small waveguides coupled to ultrahigh frequency tubes,although the short can also be used for'larger size waveguides. f

In Fig. 3 the piston 3' is shown of rathernarrowwidth and is notprovided with any bearing support whatsoever. In this case the spring 9'is shown in the form of a metallic ribbon which affords even bettercontact with the upper and lower walls of the guide than that providedwith the round wire spring .9. In Fig. .3 the spring- 9' serves as abearing support forthe piston 3" and preferably the loops of the spring.9 are fixed within the aper tures provided on the lip extension 27'.Aqtuning nut 14 is shown threaded upon the rod 6' connected to pis: ton3. The tuning nut 14 has a reduced portion 15 em gaging theend of theguide 1 so that by turning the nut 14 the rod 6 and hence the piston 3'can be adjusted longitudinally within the guide 1'. V

In Figs. 4 and the 'inventionis shown applied for use in a circular typeof waveguide. In these figures the circular guide 16 isadapted toreceive the piston 17 of cylindrical shape which piston is shownprovided with an insulating annular bearing 18 for engaging the innersurface of the guide 16. The piston 17 is provided with a forwardlyextending annular lip 19 provided circum ferentially with a number ofapertures 21 through'which a coil spring 20 is threaded, the said springbeing adapted to engage the inner wall of the waveguide. If desired, thespring 20 may have its successive loops fixed firmly within theapertures 21 thereby obtaining an even better contact pressure againstthe inner walls of the guide. Longitudinal movement of the rod 22 servesto vary the position of the short within the guide.

- In the form of the structure shown in Fig. 6 the coil is shown in theform of a flat ribbon 24 of substantially oval shape and the piston-23carrying this ribbon is shown as of relatively narrow cross-section. Theuse of the flattened spring 24 provides a very definite short circuitand at the same time use of the ribbon instead of round wire provides abetter contact with the inner walls of the guide 25.

In all forms of the invention .thecoil springstructure provides a firmcontact with the walls 'of the waveguide which is permanent in natureand will not yield as time goes on. Also as above pointed out, sincetheloops of the coil are substantially parallel to the electric vector ofthe propagated energy an efiective short circuit is provided.

Since many changes could be made in the above construction of the noveltunable waveguide short of this invention and many apparently widelydifferent embodiments of this invention could be made without departingfrom the scope thereof, it is intended that all matter contained intheabove description or shown in the accompanying drawings .shall beinterpreted as illustrative and not in a limiting sense. I

What is claimed is: y,

1. A tunable waveguide short comprising .a conducting waveguide, apiston member movable longitudinally within the waveguide, said pistonmember having a crosssectional area slightly less than that of thecross-sectional area within the waveguide, a forwardly projecting lip onthe forward end of said piston having a seriesof apertures therein, acoil spring'of conducting material mounted on the lip,.the loops of saidcoil passing through the apertures and engaging the walls ofthewaveguide to provide an effective short ,for electromagnetic energytherein, and non-conducting bearing means embedded in said piston memberand engaging the inner walls of the waveguide, said bearing meansserving as a sliding contact between said piston member and the innerwalls of the waveguide.

2. A waveguide short as claimed in claim 1 wherein said piston member,bearings, lip and coil spring are positioned longitudinally along saidwaveguide such that thesdistance between the contacting surface of saidcoil and the forward edge of said bearing, the distance between theforward edge of said bearing and the rear edge of saidbearing', andthedistance between the rear edge of said bearing and the rear edge of thepiston member are all substantially equal to a quarter wavelength of theoperating frequency of the guide such that a three section low passfilter is formed behind the short.

3. A'tunable waveguide short as claimed in claim 1 wherein said coilspring is of ribbon cross-sectional configuration.

4. A tunable waveguide short comprising a rectangular conductingwaveguide, a rectangular piston member movable longitudinally within,the waveguide, said piston member having a cross-sectional area slightlyless than that of the cross-sectional area within the waveguide, aforwardly projecting lip on the forward end of said piston havingaseriesof apertures therein, said lip extendingalong thewidcst:direction of the forward surface of the rectangular pistonmember, a coil spring of conducting material mounted on the lip, theloops of said coil passing through theapertures'and engaging the wallsof the waveguide to provide an etfective short for electromagneticenergy therein, and non-conducting bearings embedded in at least twowalls of said piston member and engaging the inner walls of thewaveguide, said bearings servingas sliding contacts between said pistonmember and the inner walls of the waveguide.

5. In combination, a'klystron including a reentrant cavity resonatorhaving a wave energy permeable opening therein and a tunable waveguideshort comprising a conducting waveguide coupled to the cavity resonatorat. said opening, a piston member movable longitudinally within thewaveguide, said piston member having a cross-sectional area slightlyless than that of the crosssectional area within the, waveguide, aforwardly projecting lip on the forward end of said pistonhaving aseries of apertures therein, a coil spring of conducting materialmounted on the lip, the loopsof said coil passing through the aperturesand engaging the walls of the waveguide to provide an effective shortfor electromagnetic energy therein, and non-conducting heating meansembedded in said piston member and engaging the inner walls of thewaveguide, said bearing means serving as a sliding contact between saidpiston member and the inner walls of the waveguide.

' References Cited in the file of this patent UNITED STATES PATENTS IGreatBritain Mar. 7,

